The present invention relates to a method for producing magnetic disk glass substrates suitable for recording media such as magnetic disks in large quantities and at a low price.
In the field of magnetic recording, particularly in magnetic disks, high performance, such as miniaturization, reduction in thickness, and high capacity has been proceeding recently, and the need for a magnetic recording medium with high density is increasing. Under such circumstances, there is much research on glass substrates because they are excellent in rigidity and hardness, easily smoothed, and extremely advantageous in increasing density and reliability.
Conventionally, glass substrates for magnetic disks were cut into predetermined sizes and polished to obtain smooth surfaces. However, in recent years, a substrate surface with super-smoothness has been demanded, and technically difficult high precision has been required in a polishing process. Therefore, polishing such substrates one by one is disadvantageous because it results in a large number of processes and an increase in the cost.
In a press molding method, a glass material is heated, molded, and cooled so that the molding face of a die is transferred to the glass material. Since this method does not require subsequent processes, it is inexpensive and provides high productivity and high quality. Therefore, the press molding method has been studied widely in the field of manufacturing optical elements and put into practical use.
However, there are different problems between forming a glass substrate having a small thickness, a large outer diameter, and a large ratio of outer diameter to thickness and forming an optical element having a relatively small ratio of outer diameter to lens thickness and a curvature.
For example, JP 1-176237 A discloses a molding method using a lens forming apparatus that includes a sleeve, a lens surface forming die sidably fitting into the sleeve, and a sleeve holder for holding the circumference of the sleeve. The method is characterized in that the amount of thermal contraction of the sleeve is smaller than that of a lens material, and the amount of thermal contraction of the sleeve holder is larger than that of the lens material.
The use of this apparatus allows a molded glass lens with a desired thickness to be obtained by specifying the thickness of the sleeve holder. Furthermore, since the amount of thermal contraction of the sleeve holder is larger than that of the lens material, pressure from an upper die always is applied to a lens during cooling, so that the lens with a surface of a desired shape can be obtained.
However, in forming such a thin glass substrate that the relationship between an outer diameter X and a thickness Y satisfies X greater than 40 Y, when a glass material is cooled under pressure of an upper die as described above, the glass material adheres to the upper and lower die and cannot be released. In recent years, the demand for the super smooth surface of a glass substrate has been especially high, and the smoother the transferring face of a die becomes, the more likely that adhesion of the glass material occurs.
For example, JP 2-26843 A discloses a method using a thin glass preform material in order to form a molded glass product having a large outer diameter and a small thickness. Since the glass preform material has the shape close to that of the molded glass product, the material is heated easily and transformed a bit. Therefore, this method can shorten a molding cycle easily.
However, when a glass substrate in the shape of a parallel plate is formed using a glass material having the above shape, in placing the glass material on a forming die, air enters the gap between the glass material and the forming die. The air cannot be removed completely even after molding, so that bubbles are generated in the glass substrate. This tendency also increases as the surface of a glass substrate becomes smoother.
It is an object of the present invention to provide a magnetic disk glass substrate with a super smooth surface that can overcome the above disadvantages of the conventional techniques and can be manufactured by press molding. It is a further object of the present invention to provide a glass substrate forming die, a glass material to be molded into a glass substrate, and a method for manufacturing a glass substrate, by which the magnetic disk glass substrate can be obtained.
A glass substrate forming die of the present invention includes a pair of upper and lower dies and a control member for controlling the space between the upper and the lower die, and is used for manufacturing a glass substrate in the shape of a parallel plate. The amount of thermal contraction of the control member is smaller than that of a material used for the glass substrate. Since the amount of thermal contraction of the control member that controls the space between the upper and the lower die in the thickness direction is smaller than that of the glass substrate material, the upper surface and/or the lower surface of the glass substrate are released from the die when cooled after molding. Therefore, a glass substrate with a super smooth surface in which the diameter is large relative to the thickness can be manufactured easily.
Materials for the control member having the amount of thermal contraction smaller than that of the glass substrate material vary depending on a glass material to be used; examples of appropriate materials are tungsten carbide, alumina, chromium, sapphire, zircon, etc.
The above effect achieved by the forming die of the present invention is remarkable particularly when the die is used for forming such a glass substrate that the relationship between an outer diameter X and a thickness Y satisfies X greater than 40 Y.
It is preferable that the control member of the forming die of the present invention controls the space between the upper and the lower die to be 1 mm or less.
Furthermore, it is preferable that at least one of the upper and the lower die is provided with a concave in the central portion thereof to specify the die used. In addition, a plurality of concaves can increase the number of types of dies to be distinguished.
Next, a glass material to be molded into a glass substrate of the present invention is used for manufacturing a glass substrate in the shape of a parallel plate with a forming die including a pair of upper and lower dies and a control member for controlling the space between the upper and the lower die. The glass material has an amount of thermal contraction larger than that of the control member. The glass material is shaped so that it comes into point-contact with the forming die when placed therein, and as the contact portion between the glass material and the forming die is increased by pressure molding, the glass material is transformed continuously so as to prevent air from entering the contact portion.
A first method for manufacturing a glass substrate of the present invention includes placing a glass material in a forming die including a pair of upper and lower dies and a control member for controlling the space between the upper and the lower die, and molding the glass material into a glass substrate in the shape of a parallel plate by pressure. The amount of thermal contraction of the control member is smaller than that of the glass material. The glass material comes into point-contact with the forming die when placed therein, and as the contact portion between the glass material and the forming die is increased by pressure molding, the glass material is transformed continuously so as to prevent air from entering the contact portion.
The glass material to be molded into a glass substrate and the first method for manufacturing a glass substrate, which are described above, allow the upper surface and/or the lower surface of the glass substrate to be released from a die when cooled after molding. This is because the amount of thermal contraction of the control member that controls the space between the upper and the lower die in the thickness direction is smaller than that of the glass material. Therefore, a glass substrate with a super smooth surface in which the diameter is large relative to the thickness can be manufactured easily.
Furthermore, the glass material comes into point-contact with the die at first, and then is molded in such a manner that the contact area is enlarged so as to prevent air from entering. Therefore, the yield of molding is good, and a glass substrate with high quality can be obtained stably. In order to achieve this, the shape of a glass material, e.g., its vertical cross section, is preferably circular or elliptical.
A second method for manufacturing a glass substrate of the present invention includes a first step of placing a glass material in a forming die including a pair of upper and lower dies and a control member for controlling the space between the upper and the lower die, a second step of heating and softening the glass material, a third step of molding the glass material into a glass substrate in the shape of a parallel plate by pressure, a fourth step of cooling the glass substrate, and a fifth step of taking out the glass substrate. The amount of thermal contraction of the control member is smaller than that of the glass material.
A third method for manufacturing a glass substrate of the present invention includes a first step of placing a glass material on a saucer, a second step of heating and softening the glass material, a third step of placing the glass material, which rests on the saucer, in a forming die including a pair of upper and lower dies and a control member for controlling the space between the upper and the lower die, a fourth step of molding the glass material into a glass substrate in the shape of a parallel plate by pressure, a fifth step of cooling the glass substrate, and a sixth step of taking out the glass substrate. The amount of thermal contraction of the control member is smaller than that of the glass material.
In the second and third manufacturing methods described above, it is preferable that application of pressure is continued until the temperature of the glass substrate reaches at least a glass transition temperature in the cooling process. This allows the form accuracy of a die to be transferred correctly to the glass substrate.
In the first, second, and third manufacturing methods described above, it is preferable that the relationship between an outer diameter X and a thickness Y of the glass substrate satisfies X greater than 40 Y. Moreover, it is preferable that the control member controls the space between the upper and the lower die to be 1 mm or less. It is preferable that at least one of the upper and the lower die is provided with a concave in the central portion thereof, and more preferably, a plurality of concaves are provided.
Furthermore, a glass substrate obtained by the above first, second, and third manufacturing methods is suitable for magnetic disks. In the case of a magnetic disk glass substrate, the glass substrate can contain bubbles that have no effect on the surface of the glass substrate.
As described above, the present invention allows a glass substrate to be obtained that has a desired surface with super-smoothness and is free of waviness by molding using a die without requiring any subsequent processes. Therefore, a magnetic disk glass substrate that is inexpensive and has a stable quality can be obtained.